Such a network consists of a collection of devices linked together by a digital bus, for example a bus according to the IEEE 1394 standard. It comprises in particular two types of device:                Source devices capable of transmitting data over the network: these devices can recover the data through a “channel” external to the network.        Receiver devices, suitable for receiving the data flowing over the network, so as to process them or present them to the user.        
Thus, taking the example of a digital home network intended for conveying audio and/or video data into various rooms of a house, the source devices are, for example, digital decoders receiving video programmes from outside the network, via a satellite antenna or via a cable connection, or else optical disc readers broadcasting over the network, in digital form, data (audio and/or video) read from a disc (in this case the disc contains data originating from outside the network). The receiver devices are, for example, television receivers making it possible to view video programmes received from the network or, more generally, any type of appliance having the capacity to decrypt encrypted data.
From the standpoint of content providers who provide the data originating from outside the local network, in particular service providers broadcasting pay-per-view televised programmes or else optical disc publishers for example, it is necessary to prevent these transmitted data from being copied and from flowing easily (for example by being copied onto an optical disc or any other recording medium) from one local network to another.
To do this, it is known practice to transmit the data in secret form by encrypting them with the aid of cryptography algorithms using keys which are known beforehand to the appliances authorized to receive these data or else which are exchanged according to particular secure protocols between the content provider and these appliances.
PCT patent application WO 00/62505 in the name of THOMSON Multimedia, filed on 31 Mar. 2000 and claiming the priority of a French patent application in the name of the same applicant, filed on 13 Apr. 1999 and published under the reference FR 2792482, relates to a domestic network in which a public key specific to the network is used to encrypt the data flowing between appliances of the network, typically from the previously mentioned source devices to receiver devices. Only the appliances of this network possess the private key corresponding to the public key. The (public key, private key) pair being specific to the network, data encrypted within the framework of this network cannot be decrypted by appliances of another network.
The use of a pair of asymmetric keys has certain advantages, but also some drawbacks. One of the main advantages is that no secret is stored in the source appliances: these appliances are aware of the public key, but not the private key. However, the implementation of asymmetric keys is relatively slow, as compared with that of symmetric keys. Moreover, the lifetime of asymmetric keys is short, requiring periodic revocation and the creation of new keys. In this case, data encrypted with a key and then recorded might suddenly no longer be decryptable on the network. Moreover, a sizeable number of pairs of asymmetric keys is necessary.
One would then be tempted to implement a symmetric key to encrypt the data. However, this would require the source devices to be aware of this key, and this would impose increased security constraints on them and consequently render them more expensive.